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 01.11.2010   Карта сайта     Language По-русски По-английски
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01.11.2010


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Acta Materialia
Volume 59, Issue 1, January 2011, Pages 102-111







Colossal dielectric constant of poly- and single-crystalline CaCu3Ti4O12 fibres grown by the laser floating zone technique





F. Amarala, b, L.C. Costab, Corresponding Author Contact Information, E-mail The Corresponding Author, M.A. Valentea, A.J.S. Fernandesa, N. Francoc, E. Alvesc and F.M. Costaa






a Physics Department and I3N, University of Aveiro, 3810-193 Aveiro, Portugal


b College of Technology and Management of Oliveira do Hospital, 3400-124 Oliveira do Hospital, Portugal


c Institute Technologic Nuclear, ITN, 2686-953 Sacavém, Lisboa, Portugal





Received 2 July 2010; 


revised 2 September 2010; 


accepted 7 September 2010. 


Available online 14 October 2010.









Abstract



The laser floating zone technique (LFZ) was used to produce CaCu3Ti4O12 (CCTO) samples with different pulling rates (Rp). Polycrystalline samples were obtained for higher Rp, and single crystals for low Rp. Morphologic characterization was done by scanning electron microscopy (SEM), while the structural analysis was accomplished using X-ray diffraction and Raman spectroscopy. The best crystallinity was obtained for the fibre grown at the lowest pulling rate, resulting in a CCTO single crystal. Dielectric spectroscopy was performed between 40 Hz and 50 MHz in the temperature range of 90–350 K. The dielectric strength varies from 3 × 104 for the fibres grown at higher Rp (80 mm h−1), to 5 × 104 for the ones grown at lower Rp (5 mm h−1). The Cole–Cole model was used to fit the dielectric data in order to calculate the relaxation parameters. The dielectric properties measured at microwave frequency (2.7 GHz) by the resonant cavity method confirm the high dielectric constant (56.7) and relatively low values of tan δ (0.04). This is a clear indication that this material is potentially interesting for microwave device applications.



The similarities between the AC activation energies of LFZ fibres and bulk CCTO, calculated from relaxation times, suggest that an additional polarization mechanism common to single and polycrystalline CCTO fibres must also be present and should contribute for the further increase of the dielectric constant.





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